1. Field of the Invention
The present invention generally relates to a method and apparatus for printing a large volume of print data including both an image for duplex printing and an image for simplex printing, without a decrease in print speed. The present invention also relates to a computer-readable recording medium in which a program for implementing the aforementioned method is recorded.
2. Description of the Related Art
Special applications are currently available that enable the generation of print data at the time of printing without the use of a printer driver. The “print data” herein includes an image to be printed and a print instruction that are transmitted to an image forming apparatus, such as a printer (The “image” herein refers to the image-rendered portion of PDL (Printer Description Language) data upon generation of the PDL data).
Such a special application may generate print data that includes both an image for which duplex printing (by which both sides of a transfer sheet are printed) is designated and another image for which simplex printing (by which only one side of a transfer sheet is printed) is designated. No such print data is generated by a printer driver. Conventionally, a dedicated printer has been employed for printing from such a special application; increasingly, however, normal office printers are employed due to improvements in their performance.
When duplex printing is performed by an image forming apparatus in order to save paper resources, for example, a transfer sheet is inverted along a duplex transport path. Accordingly, particularly in the case of a large-sized image forming apparatus with a long transport path, a time lag is produced between the completion of printing of an upper surface of the transfer sheet and the start of printing of a back surface of the transfer sheet when the upper surface and the back surface are printed alternately, resulting in a reduction in print speed and lowered productivity.
In order to solve this problem, the image forming apparatus may be provided with an interleave function. The interleave function improves the productivity of duplex printing by repeating the printing of upper surfaces of more than a certain number of transfer sheets, and then printing their back surfaces (see Japanese Laid-Open Patent Application No. 2003-173115, for example).
However, the interleave function may lead to lowered productivity in some cases of duplex printing. Specifically, when an original document is printed using the aforementioned special application that generates print data that includes both images for duplex printing and images for simplex printing, print speed sometimes significantly suffers compared to cases of duplex printing alone or simplex printing alone, particularly in the case of a high-speed image forming apparatus. One of the causes of this problem is the presence of a sheet interval. Upon switching from duplex printing to simplex printing in an image forming apparatus, the transfer sheet transport path is also switched from a duplex transport path to a simplex transport path. Depending on the number of transfer sheets that are being transported in the image forming apparatus, a sheet interval, which is a blank interval where no printing is performed, may be produced between the transfer sheets on the duplex transport path.
The sheet interval means an inactive period for a printer engine and thus a decrease in print speed when printing a large volume of print data including both duplex printing and simplex printing data.